1. Field of the Invention
The present invention relates to an injection molding machine driven by a electric motor, such as a servomotor and, more particularly, to a motor control method and a motor controller for an injection molding machine to avoid an overloaded or overheated operating condition of the motor.
2. Description of the Related Art
A motor used as a driving source for an injection molding machine drives a screw of the injection molding machine for injecting the molten material into a mold, holding pressure in the mold, and applying back pressure to the molten material during charging process. During an operation for injecting or applying back pressure, the output torque of the motor can be decreased and the magnitude of the drive current applied to the motor is relatively low. But, during holding pressure phase, the output torque of the motor should be increased and the motor demands a very high driving current.
A motor of this kind is selected mainly from the economical point of view, taking design requirements into consideration. Generally, it is rare to adopt a large capacity motor with a continuous rating current enough to exceeded driving current greatly. Therefore, in some cases, the motor demands a driving current greatly exceeding its continuous rating current during the pressure holding operation, and the mean value of the driving current supplied to the motor in one injection molding cycle exceeds the continuous rating current. Since the amount of heat generated by the motor is proportional to the square of the driving current, it is possible that the motor overheats when the mean value of the driving current supplied to the motor in one injection molding cycle exceeds the continuous rated current. A motor control method for an injection molding machine to prevent the overheating of a motor included in the injection molding machine is proposed in JP-B No. 1992-46215. This prior art motor control method comprises steps of measuring the total amount of heat actually generated by the motor in one injection molding cycle of the injection molding machine, comparing the total amount of heat actually generated by the motor with a rated amount of heat that will be generated by the motor if a current equal to the continuous rating current of the motor is supplied to the motor, deciding that the motor is overheating when the total amount of heat actually generated by the motor exceeds the rated amount of heat, and carrying out an overheat avoiding operation if it is decided that the motor is overheating.
Another motor control method for an injection molding machine is proposed in JP-B No. 1992-51332. This prior art motor control method for an injection molding machine comprises steps of sampling a current supplied to the motor at predetermined intervals in one injection molding cycle, calculating the ratio between the sum of the products each of the square of a measured current value and the sampling interval, and the product of the square of a maximum current that can be supplied to the motor and the period of the injection molding cycle, deciding whether or not the motor is overheating on the basis of the ratio, and suspending an injection molding operation to extend the period of the injection molding cycle if it is decided that the motor is overheating.
In the prior art mentioned above, a servomotor with continuous rating is adopted. The servomotor is provided with a sensor for measuring the temperature of the servomotor to protect the servomotor from overheating.
Although the injection molding cycle needs to be repeated without interruption so as to produce molded products continuously, the servomotor is stopped to interrupt the injection molding cycle when the temperature of the servomotor measured by the sensor exceeds an upper limit and a motor protecting function is actuated. The interruption of the injection molding cycle often incurs increase in production costs and decrease in productivity.